A photodetector (PD) is a device configured to detect an optical signal and convert the optical signal into an electrical signal. In particular, an ultraviolet (UV) PD has widely been used for various purposes in medical, industrial, and military fields. The UV PD is typically fabricated using silicon (Si). However, since silicon has a small energy bandgap, the lifespan of a device is shortened due to energy of visible (V) light, which is incident in addition to UV light. Also, a PD fabricated using silicon requires an additional filter configured to filter V light and infrared (IR) light.
To solve these problems, active research has been conducted on a technique of fabricating a PD using a material having a large energy bandgap, such as gallium nitride (GaN) and zinc oxide (ZnO). In particular, since ZnO has a large energy bandgap of about 3.35 eV, no additional V light filter is required. Also, since ZnO uses a material having low toxicity with respect to the human body, such as zinc, an environmentally friendly device may be fabricated. Various structures of UV PDs using ZnO have been studied. Most typically, a structure using a ZnO thin film, which is similar to a conventional PD, has been used. However, when a PD is fabricated using a ZnO thin film, it is difficult not only to grow a high-quality thin film capable of preventing current leakage but also to obtain a low-cost substrate having the same lattice constant as the ZnO thin film.
Due to the problems of the ZnO thin film, research on fabrication of a UV PD using ZnO structures, such as ZnO nanoparticles and ZnO nanowires, has been attempted. When the ZnO nanowires are used, the ZnO nanowires are disposed across a space between two electrodes. When light is incident, a depletion layer present in the ZnO nanowires is reduced due to electron-hole pairs formed in the ZnO nanowires, thereby increasing the electrical conductivity of the ZnO nanowires. When the ZnO nanoparticles are used, the ZnO nanoparticles are used as an electron transport material. Also, when light is incident, a depletion layer present in the ZnO nanoparticles is reduced due to electron-hole pairs formed in the ZnO nanoparticles, thereby increasing electrical conductivity.
A method of fabricating a PD using ZnO nanowires has high charge transport efficiency because the nanowires have a higher electrical conductivity than nanoparticles. However, the method has low operating efficiency because an effective area to which light is incident is smaller than a method using ZnO nanoparticles. Also, since it is very difficult to align the nanowires between specific electrodes, commercial production of PDs is not possible. To solve this problem, a method of adhering the ZnO nanoparticles to the ZnO nanowires may be used. However, a process of adhering the ZnO nanoparticles to the ZnO nanowires is complicated, and forming electrodes in the nanowires or forming the nanowires between the specific electrodes is still difficult.
In a process of fabricating a device using only ZnO nanoparticles, since a material for structurally fixing the nanoparticles is required, there are few cases where only the nanoparticles are used. In most cases, a structure in which the ZnO nanoparticles are included in a polymer is used. A method using a polymer including the ZnO nanoparticles may employ a process using a solvent, such as a spin coating process. However, the method using the polymer including the nanoparticles requires a transparent electrode or a transparent substrate as a stacked structure and further requires additional hole and electron transport layers in addition to a polymer layer configured to fix the nanoparticles.
As a method of fabricating a device using only ZnO nanoparticles without an additional matrix material, such as a polymer, a process of sintering the ZnO nanoparticles using an annealing process for fixing the ZnO nanoparticles may be considered. However, since the nanoparticles cling to one another during the sintering process, the device cannot be flexible. Also, since a high-temperature annealing process is required, a polymer substrate vulnerable to heat cannot be used. Furthermore, since a high voltage of about several tens of volts or higher should be applied due to a low electrical conductivity between the nanoparticles, the device cannot be used as a portable device.